This invention relates to a technology for communicating a quantity of information of a digital signal, which is suitable in converting an acoustic signal or any other analog signal to a digital signal to record or transmit such a digital signal.
In recent years, analog signals have been frequently recorded or transmitted as a digital signal, e.g., a pulse code modulation (PCM) signal. Since the PCM signal has a large quantity of information, a broad transmission band for recording or transmission thereof is required. In view of this, a scheme has been conventionally adopted in recording/reproducing equipment, transmitting equipment or other various equipment for carrying out signal processing of a digital signal to efficiently process digital signals using a lesser quantity of information.
As an efficient coding system capable of efficiently coding signals using a lesser quantity of information, there have been conventionally proposed various systems of the type in which prediction of signals carried out to record or transmit only a component of deviation from a predicted value (residual component), or various system of the type in which a sort of transform (generally, orthogonal transform) processing is implemented to a signal to extract the feature of that signal to lessen a quantity of information (the number of bits) at every respective sample by making use of the property that the feature portion of that signal or the visual sense or the hearing sense of the human being is sensitive to a change at the portion where a change of a signal is small, but is not sensitive at the portion where a signal greatly changes even if there is an error to some extent, or the like.
FIG. 1 is a block diagram showing a well known efficient coding system constructed by applying thereto a linear prediction put into practice for the purpose of compressing a quantity of information in transmitting a speech signal of a telephone. In the efficient compression system shown in FIG. 1, numerator (zero) and denominator (pole) of the predictive system are predicted. However, its prediction ability is not so satisfactory, and is hardly effective in transmission of an acoustic signal. As the efficient coding system by the linear prediction, the Parco system exists in addition to the above-mentioned system, but has a limitation in performance.
Since, as indicated above, there has been no predicting method suitable for compression of a quantity of information relating to recording or transmission of an acoustic signal, the efficient coding method of the bit compression type has been most widely used for the purpose of compression of a quantity of information relating to recording or transmission of an acoustic signal. In this type, as shown in the flow chart of FIG. 2(a), for example, bit compression is carried by making use of the masking effect between nearly frequencies provided by orthogonal transform processing. Namely, an approach is employed as shown in FIG. 2 to extract, from an acoustic signal, periods, e.g., having 1024 sampling points as periods of respective sequential frames with their connecting portions slightly overlapping with each other by making use of a window function to carry out orthogonal transform processing by Fast Fourier Transform (FFT) at every one of the frames.
Then, a frequency value having the largest amplitude is determined from data obtained by the FFT to perform a calculation for determining a masking curve as shown in FIG. 2(c) with the above-mentioned frequency being a center. On the basis of such a masking curve, a processing is conducted such that a spectrum having an amplitude larger than the masking curve is recorded or transmitted, and a spectrum having an amplitude smaller than the masking curve is not recorded or transmitted. In particular, processing is conducted such that a corresponding sound which is inaudible is not recorded or transmitted. As the orthogonal transform technique, a discrete Fourier transform (DFT) or discrete cosine transform (DCT), etc. may be used.
The reason why data can be reduced as stated above is as follows. Namely, in the case where sound of a certain frequency component is radiated at a high intensity, a human being's ability to sense frequencies in the vicinity of that frequency component is lowered. Therefore, means are employed such that a lesser number of bits are allocated to the frequency component at the portion where the sensing ability is lowered, and such that signal components having a small amplitude are not transmitted at all, or other means, thereby making it possible to realize a reduction in the quantity of data. While the signal accuracy is lowered to a considerable degree by the reduction in the quantity of data as stated above, a measure is frequently taken such that no distinction or difference exists between hearing a signal in which the data quantity is reduced and hearing an original signal by the masking effect in the hearing sense.
As described above, since prediction of a signal and the orthogonal transform processing of a signal have been independently carried out for carrying out efficient coding in the prior art, sufficiently efficient coding could not be conducted.